1. Field of the Invention
The present invention relates to a color image forming apparatus. More specifically, the invention relates to a color image forming apparatus which forms latent image on a surface of a photoconductor and forms a multi-color or monochrome image on a printing medium.
2. Description of the Related Art
As such color image forming apparatus, there have been conventionally proposed and practiced several systems.
Typical one of the conventional systems is an apparatus, in which developing devices containing developers of mutually different colors are arranged with respect to a photoconductor for sequentially effecting development, respective colors of developer images formed on the photoconductor are overlaid on a printing medium or an intermediate transfer medium, in case of directly overlaying on the printing medium, the overlaid image on the printing medium is subsequently fixed, and in case where the images are overlaid on the intermediate transfer medium, the images overlaid on the intermediate transfer medium is transferred on the printing medium in batch process, and then fixed.
On the other hand, there is an apparatus, in which respective colors of developer images are selectively overlaid sequentially directly on the photoconductor to form a color developer image on the surface of the photoconductor, subsequently transferred the formed image in batch process, and then fixed.
There has been further proposed an apparatus called as a tandem type apparatus, in which a plurality of photoconductors are employed so that respective photoconductors share respective developer images of respective colors, the images formed on respective photoconductors are sequentially transferred on the printing medium, and then the image formed on the printing medium is fixed. There has been also proposed an apparatus, in which instead of directly transferring image on the printing medium by feeding the printing medium, the image is transferred via the intermediate transfer medium so as to avoid degradation of the printed image due to kind of the printing medium and/or variation of characteristics of the printing medium as much as possible.
Furthermore, in case of the method to directly form the color image on the photoconductor, there has been proposed an apparatus, in which a plurality of charging means, exposure means and developing devices having mutually different colors of developers are arranged in sequential order.
The conventional color image forming apparatus set forth above respectively encounter the following problems.
At first, in case of the apparatus which employs a method to sequentially form respective color of developer images with a single photoconductor and to sequentially overlay the developer images in sequential order, since only one photoconductor and exposure light source are used, it becomes necessary to repeat image forming operation for three or four times in order to form three or four colors of images which are required for forming the color image. Therefore, throughput in color image printing becomes low. Furthermore, for single photoconductor, respective colors of developing devices are switched, mechanical vibration tends to be caused to result in difficulty in obtaining high quality image output. Also, for arranging respective colors of developing devices on the circumference of the photoconductor, large photoconductor becomes necessary to make the overall image forming apparatus bulky and to make the production cost high in consideration of relatively low output speed.
Next, in the apparatus which employs a method to directly form the color image on the photoconductor, the photoconductor is required to be a size corresponding to a possible maximum printing medium size to be used for outputting the color image. Therefore, the photoconductor becomes bulky to results in high production cost. Furthermore, similarly to the former prior art, since the printing operation has to be repeated for three or four times, color image output speed becomes low.
In case of the tandem type color image forming apparatus, in comparison with the apparatus employing single photoconductor, a plurality of optical systems are required for exposure to form latent images on respective photoconductors. Therefore, the apparatus becomes complicate and high in production cost. Furthermore, multi-color output speed and monochrome output speed becomes equal to lowering efficiency of monochrome output which is frequently used in general, to be about one third to one fourth.
Therefore, in Japanese Unexamined Patent Publication (Kokai) No. Heisei 4-230774, for example, there has been proposed an apparatus, in which a plurality of laser beams are deflected by a polygon reflector body to guide respective laser beams to a plurality of photoconductors to perform exposure. However, in this proposal, since a plurality of laser beams are employed, it causes drawback in high cost for requirement of a plurality of lasers, as well as in difficulty of optical adjustment.
Also, in the apparatus which employs a method to sequentially arrange a plurality of charging means, exposure means and respective colors of developing devices along the circumference of single photoconductor, the photoconductor becomes quite large. Furthermore, similarly to the tandem type, in order to realize the optical system for exposure for obtaining a plurality of exposure beams to cause significant rising of the production cost.
In view of this, Japanese Unexamined Patent Publication No. Heisei 5-80631 proposes an apparatus, in which number of polygon reflector bodies is reduced to be half, and polygon reflector bodies are mounted on a single motor and whereby a production cost required for such optical system for exposure can be lowered. However, since number of the lasers required corresponds number of developing colors, the production cost is still high. Also, since a large size photoconductor belt is employed, the production cost becomes high.
On the other hand, Japanese Unexamined Patent Publication No. Showa 63-143570 or Japanese Unexamined Patent Publication No. Showa 63-143571 proposes an apparatus, in which a single polygon reflector body is employed, a plurality of inclination angles are provided for a plurality of reflector surfaces, the laser beam distributed to respective reflector surface to scan the photoconductor at a plurality of exposure positions for exposure. However, even with this, the speed of monochrome image output becomes equal to that in multi-color printing.